Piezoelectric actuators, which drive an object by utilizing the deformation of a piezoelectric material when electric field acts thereon, have been hitherto used widely in various fields. For example, U.S. Pat. No. 6,891,314 B2 (corresponding to Japanese Patent Application Laid-open No. 2003-69103) shows in FIGS. 1 to 3 an ink-jet head provided with a piezoelectric actuator which applies discharge pressure to ink. The piezoelectric actuator includes a metallic substrate supported by rigid components, a plurality of piezoelectric elements (piezoelectric layers) arranged on the substrate, signal electrodes each of which is formed in upper surface of one of the piezoelectric elements, and common electrodes each of which is formed in the lower surface of one of the piezoelectric elements. Each of the piezoelectric elements has a deformation area (actuator area) at which the substrate is not joined or bonded to one of the rigid components and a joining area (area for electric connection) at which the substrate is joined to one of the rigid components. In each of the piezoelectric elements, the signal electrode is extended from the deformation area to the joining area, at which the signal electrode is electrically connected to a flexible print circuit (FPC) board formed of a flexible resin base material. Driving signal is supplied to each of the signal electrodes via this FPC.
Here, in the surface of the FPC on the side of (facing) the piezoelectric elements, terminals are formed to correspond to the signal electrodes respectively. Each of the terminals has a core and a semi-spherical bump which covers the core and which is made of electrically conductive joining material. The process for connecting the FPC to the signal electrodes involves positioning each of the bumps of the FPC and the portion of associated signal electrode, the portion being extended to the joining area of the signal electrode. In this state, the bumps are heated to melt the joining material, thereby joining, with the molten joining material, the signal electrodes and the terminals of FPC respectively.
In the piezoelectric actuator described in U.S. Pat. No. 6,891,314 B2, when the joining material is melted to join the signal electrodes and the terminals of the FPC, a portion of the molten joining material flows out to the surroundings. At this time, when the joining material flows along the signal electrode up to the deformation area of the associated piezoelectric element, then the deformation of the piezoelectric element at its deformation area is hindered by the joining material, which results in preventing the normal operation of the piezoelectric actuator. In addition, there is a feat that the molten conductive joining material flows from a certain signal electrode to another signal electrode adjacent to this signal electrode, causing a short circuit between the two signal electrodes.